GCIB-treated resistive device

ABSTRACT

The present disclosure includes GCIB-treated resistive devices, devices utilizing GCIB-treated resistive devices (e.g., as switches, memory cells), and methods for forming the GCIB-treated resistive devices. One method of forming a GCIB-treated resistive device includes forming a lower electrode, and forming an oxide material on the lower electrode. The oxide material is exposed to a gas cluster ion beam (GCIB) until a change in resistance of a first portion of the oxide material relative to the resistance of a second portion of the oxide material. An upper electrode is formed on the first portion.

PRIORITY APPLICATION INFORMATION

This application is a Continuation of U.S. application Ser. No.13/551,243, filed Jul. 17, 2012, which is a Continuation of U.S.application Ser. No. 12/693,936, filed Jan. 26, 2010, issued on Jul. 17,2012 as U.S. Pat. No. 8,223,539, the specifications of which areincorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to the field of semiconductordevices. More particularly, in one or more embodiments the presentdisclosure relates to a gas cluster ion beam (GCIB) treated resistivedevices and methods of forming a GCIB-treated resistive device.

BACKGROUND

Resistive devices can be used as semiconductor resistors, switches, ormemory elements (e.g., memory cells of a memory device), among otherapplications. Memory devices are typically provided as internal,semiconductor, integrated circuits in computers or other electronicdevices. There are many different types of memory includingrandom-access memory (RAM), read only memory (ROM), dynamic randomaccess memory (DRAM), synchronous dynamic random access memory (SDRAM),flash memory, and resistive random access memory (RRAM), among others.

Memory devices are utilized as non-volatile memory for a wide range ofelectronic applications in need of high memory densities, highreliability, and low power consumption. Non-volatile memory may be usedin a personal computer, a portable memory stick, a solid state drive(SSD), a personal digital assistant (PDA), a digital camera, a cellulartelephone, a portable music player (e.g., MP3 player), a movie player,and other electronic devices, among others. Program code and systemdata, such as a basic input/output system (BIOS), are typically storedin non-volatile memory devices.

Memory cells can be arranged in a matrix (e.g., an array). For example,an access device (e.g., transistor) of a number of memory cells may becoupled to an access line (one example of which is a “word line”)forming a “row” of the array. The memory elements of each memory cellare coupled to a data line (one example of which is a “bit line”) in a“column” of the array. In this manner, the access device of a memorycell is accessed through a row decoder activating a row of memory cellsby selecting the word line coupled to their gates. The programmed stateof a row of selected memory cells is determined by causing differentcurrents to flow in the memory elements depending on the resistanceassociated with a programmed state for a particular memory cell.

Memory cells can be programmed (e.g., write, erase) to a desired state.That is, one of a number of programmed (e.g., resistance) states can beset for a memory cell. For example, a single level cell (SLC) canrepresent one of two logic states (e.g., 1, 0). Resistive memory cellscan also be programmed to one of more than two programmed states, suchas to represent more than two binary digits (e.g., 1111, 0111, 0011,1011, 1001, 0001, 0101, 1101, 1100, 0100, 0000, 1000, 1010, 0010, 0110,1110). Such cells may be referred to as multi state memory cells,multi-digit cells, or multilevel cells (MLCs).

Non-volatile resistive memory such as RRAM stores data by varying theresistance of a resistive memory element. Data may be written to aselected memory cell in an RRAM by applying a predetermined voltage, ata predetermined polarity, for a predetermined duration, to a particularresistive element. RRAM can be programmed to a number of resistancestates by application of voltage of various magnitudes, polarities,and/or durations.

One type of resistive memory element is a memristor. Memristors can beused to form RRAM. Such an RRAM can be formed of a material that can beconfigured to provide variable resistance, such as an oxide (e.g., metaloxide such as a transition metal oxide (TMO), nitrides, etc.). The RRAMmay utilize a resistance transition characteristic of the TMO by whichresistance of the material varies according to a change in applicationof voltage. Memristors can be implemented in nanoscale devices, therebyenabling storage elements to provide a high density, low cost,non-volatile, high speed RAM without the read/write cycle endurancelimitations of charge-storage type memory.

A resistive device can have an active region that is formed of one ormore materials that are electronically semiconducting (e.g., nominallyelectronically insulating) and also are weakly ionic conductor(s).Material(s) of the active region can be capable of hosting andtransporting ions that act as dopants to control the flow of electronsthrough the material(s). Ionic transport may also be understood as thetransport of the absence of a particular ion (e.g., ionic vacancies),similar to understanding electric current by the movement of “holes”representing the absence of an electron. That is, ionic vacancies appearto move in a direction opposite to that of the corresponding ions.

According to one previous approach, the active region of a resistivedevice is formed by depositing two discrete materials that differ insome initial characteristic (e.g., concentration of ionic vacancies).Operation of the resistive device involves transport of ionic vacanciesfrom the first region, across a boundary between the two discreteregions, to the material of the second region. The active region thuscomprises, for example, a primary material for transporting and hostingions that act as dopants to control the flow of electrons, and asecondary material for providing a source of ionic dopants for theprimary material. However, the physical boundary between the two regionsof material that differ in some initial characteristic can result insome undesirable consequences.

One limitation of one or more previous resistive device fabricationapproaches is an inability to control small changes in atomic andvacancy arrangement during the film stack creation, and to not damagethe thin film stack during patterning. Previous methods for creatingoxides from metals also tend to be grain boundary sensitive. Grainboundaries can result from a lack of fabrication control while forming aplurality of discrete regions (e.g., materials). Due to small featuresize limitations (e.g., sub 20 nm), direct deposition methods arelimited.

According to another previous approach, the active region of a resistivedevice is formed by depositing one material (e.g., a material), andusing a voltage (e.g., 5-10 V) in forming the resistive device that isstronger than the electric field (e.g., 2-2.5 V) that is used tothereafter operate the resistive device to form two regions within theactive region that differ in some characteristic (e.g., concentration ofionic vacancies). The two regions can form a gradient, with one end ofthe gradient acting as a first “region” and an opposite end of thegradient acting as a second “region.” However, the application of astrong electric field, perhaps for an extended period of time, ininitially forming the two regions with one material can result in someundesirable consequences. For example, application of a voltage largeenough to initially form the two regions (e.g., 5-10 V) can causedielectric leakage and/or change the threshold of the dielectric.

Furthermore, applying a higher voltage bias to resistance device causesboth electron current and ion current to flow, whereas at a lowervoltage bias the flow of ion current is negligible, which allows theswitch to hold its resistance state. Therefore, where a strongelectrical field is used initially in forming two regions, the highervoltage bias can cause unintended ions in the molecular structure tomove (e.g., sodium ions can become mobile and move as the oxygenvacancies are being moved). For these and other reasons, a method forforming two regions (e.g., portions) of an active region, the regionsbeing defined based on ionic species concentration (e.g., of oxygenvacancies), without having to move the ionic species using relativelystrong electric fields, would be advantageous.

One previous mode of operating the resistive device after formation isto apply a relatively large electrical field across the resistive devicethat exceeds some threshold for enabling the motion of the ions (orvacancies) in the material(s) to cause an ionic species (or vacancythereof) to be transported via ionic transport (e.g., into or out of thefirst material from/to the second material, or into or out of a firstregion from/to a second region initially formed of similar material andsubsequently altered by application of a strong electric field).

Accordingly, a method for forming a resistive device without thedisadvantages associated with the deposition of two separate materials(e.g., grain boundaries) or the application of a strong electrical fieldfor initially in forming two regions would be beneficial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram of a non-volatile memoryimplemented using a gas cluster ion beam (GCIB)-treated resistive devicein accordance with one or more embodiments of the present disclosure.

FIG. 2A is a perspective view of a GCIB-treated resistance device beingoperated to a first resistive state in accordance with one or moreembodiments of the present disclosure.

FIG. 2B is a perspective view of a GCIB-treated resistance device beingoperated to a second resistive state in accordance with one or moreembodiments of the present disclosure.

FIG. 3A is a diagram illustrating one example physical layout of aGCIB-treated resistance device in accordance with one or moreembodiments of the present disclosure.

FIG. 3B is an equivalent circuit diagram illustrating variableresistance of a GCIB-treated resistance device in accordance with one ormore embodiments of the present disclosure.

FIGS. 4A-4D are cross-sectional views illustrating a process for forminga GCIB-treated resistance device in accordance with one or moreembodiments of the present disclosure.

FIGS. 5A-5D are cross-sectional views illustrating a process for forminga GCIB-treated resistance device including growth of a metal oxide filmin accordance with one or more embodiments of the present disclosure.

FIG. 6A-6C are cross-sectional views illustrating a process for forminga GCIB-treated resistance device including partial growth of a metaloxide film in accordance with one or more embodiments of the presentdisclosure.

FIGS. 7A-7E are cross-sectional views illustrating a process for forminga GCIB-treated resistance memory including a recessed active region inaccordance with one or more embodiments of the present disclosure.

FIG. 8 illustrates an example stacked RRAM structure having multipleresistance states that can be implemented as the memory element in FIG.1 according to one or more embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure includes GCIB-treated resistive devices, devicesutilizing GCIB-treated resistive devices (e.g., as switches, memorycells), and methods for forming the GCIB-treated resistive devices. Onemethod of forming a GCIB-treated resistive device includes forming alower electrode, and forming an oxide material on the lower electrode.The oxide material is exposed to a gas cluster ion beam (GCIB) until achange occurs in resistance of a first portion of the oxide materialrelative to a second portion of the oxide material. An upper electrodeis formed on the first portion.

The figures herein follow a numbering convention in which the firstdigit or digits correspond to the drawing figure number and theremaining digits identify an element or component in the drawing.Similar elements or components between different figures may beidentified by the use of similar digits. For example, element “02” maybe referenced as 102 in FIG. 1, and a similar element may be referencedas 202 in FIG. 2.

The ionic species are specifically chosen from those that act aselectrical dopants for the material(s), and thereby change theelectrical conductivity of the material(s) from lower conductivity of anundoped semiconductor (e.g., insulator) to a higher conductivity of adoped semiconductor. One having ordinary skill in the art willappreciate that the resistive device may thus act as a switch using thelower conductivity state (e.g., higher resistive state) as a switch-ONconfiguration, and the higher conductivity state (e.g., lower resistivestate) as a switch-OFF configuration. One having ordinary skill in theart will further appreciate that the resistive device may thus act as amemory element with one resistive state corresponding to a first logicstate, and another resistive state corresponding to a another logicstate.

FIG. 1 is a functional block diagram of a non-volatile memory 100implemented using a resistive device 102 in accordance with one or moreembodiments of the present disclosure. The resistive device can includea gas cluster ion beam (GCIB) infused region 104 (e.g., a first region)and second region 106, formed between a first electrode 105 and a secondelectrode 107. The GCIB-treated resistive device 102 can have a firstterminal 114 and a second terminal 118. While FIG. 1 shows theGCIB-treated resistive device 102 being formed to have only two regions(e.g., first region 104 and second region 106), embodiments of thepresent disclosure are not so limited. For example, embodiments of thepresent disclose may be implemented with additional regions, materials,configurations, and/or features, which are omitted from FIG. 1 forclarity.

An access device (e.g., transistor) 110 is coupled in series with theGCIB-treated resistive device 102 to form a memory cell 112. Accessdevice 110 serves as a switch for enabling and disabling current flowthrough the GCIB-treated resistive device 102. Access device 110 may be,for example, a complementary metal oxide semiconductor (CMOS) transistorwith a gate coupled to a word line 124. However, embodiments of thepresent disclosure are not limited to any particular type of accessdevice, and can be implemented using other types of switching devices.Thus, when word line 124 voltages are applied to activate/deactivateaccess device 110 to appropriately complete the circuit between a sourceline 122 and a bit line 120 through the GCIB-treated resistive device102. As shown, memory cell 112 is coupled to the bit line 120 by thefirst terminal 114, and coupled to the source line 122 by the secondterminal 116.

According to one or more embodiments, the bit line 120 and source line122 are coupled to logic for reading from, and logic for writing to, thememory cell 112. For example, a read/write control multiplexer 130 hasan output coupled to the bit line 120. The read/write controlmultiplexer 130 is controlled by a read/write control logic line 132 toselect between a first input coupled to a bipolar write pulse generator126, and a second input coupled to a read sensing logic 128. The amount,polarity, and/or duration of voltage potential applied to the memoryelement 102 for programming may be controlled by application thereofbetween the bit line 120 and the source line 122 associated with theselected memory cell 112.

According to one or more embodiments, during a read operation, a biasgenerator 129 establishes (through the read sensing logic 128) a readbias voltage potential difference between the bit line 120 and thesource line 122 (e.g., a fixed voltage) associated with the selectedmemory cell 112. The read bias voltage causes a particular magnitude ofcurrent to flow corresponding to the resistance of the GCIB-treatedresistive device 102 (e.g., the greater the resistance of theGCIB-treated resistive device 102, the smaller the current that flowsfor a given read bias voltage according to Ohm's law). The amount ofcurrent flowing through the GCIB-treated resistive device 102 during aread operation (or a voltage proportional thereto) can be sensed by theread sensing logic 128 (e.g., a sense amp may compare a circuit-derivedinput to a reference input that corresponds to a boundary conditionbetween two programmed states) to determine an output corresponding tothe programmed state represented by the present resistance of theGCIB-treated resistive device 102.

According to one or more embodiments, a read current is applied throughthe GCIB-treated resistive device 102 causing a corresponding voltage tobe developed, which can be sensed and compared to a reference voltage.From the comparison, the resistance of the memory element may bedetermined (e.g., based on the principles of Ohm's law).

Although FIG. 1 illustrates, and the discussion above describes, amemory cell 112 including one configuration of a GCIB-treated resistivedevice 102, and particular read and write logic arrangement, one havingordinary skill in the art will appreciate that one or more embodimentsof the present disclosure may be implemented using other configurationsof a GCIB-treated resistive device and/or other configurations of logicfor switching and/or programming the GCIB-treated resistive device 102.

RRAM can include types of RRAM in which different data values may bewritten in accordance with the polarity of the applied voltage throughthe GCIB-treated resistive device 102. Such devices are sometimesreferred to as a “bipolar RRAM.” In the case of a bipolar RRAM, a bitline and source line can be used with each memory cell in order to writedifferent data values to the bipolar RRAM.

FIG. 2A is a perspective view of a GCIB-treated resistance device beingoperated to a first resistive state in accordance with one or moreembodiments of the present disclosure. In the embodiment illustrated inFIGS. 2A-3A, the active region (e.g., 203 in FIGS. 2A-2B) is initiallyformed using a thin film of oxide material (e.g., TiOx, NiOx, CuOx,AlOx) and/or nitride material (e.g., metal nitrides such as aluminumnitride). The following operational discussion is based on use ofstoichiometric TiO₂ to form an active region, with the moving ionicspecies being oxygen vacancies. However, embodiments of the presentdisclosure are not so limited, and may for example, be fabricated frommetal nitrides with the moving ionic species being nitrides.

The stoichiometric TiO₂ active region is subsequently exposed to a gascluster ion beam to form a first region (e.g., first portion of theactive region) and a second region (e.g., second portion of the activeregion), the regions being defined based on location of oxygen vacanciesin the molecular structure of the initial TiO₂. Additional fabricationdetails for various GCIB-treated resistance devices are discussedfurther with respect to FIGS. 4A-7E below.

FIG. 2A illustrates the GCIB-treated resistance device 202 beingoperated to obtain a high conductivity state (e.g., low resistancestate). To obtain the low resistance state, positively charged donors,such as oxygen vacancies in TiO₂, are moved about the active region 203,being driven from the first region 204 into the second region 206 byapplication of a positive voltage 213 across the GCIB-treated resistancedevice 202 that exceeds the threshold electric field for the drift ofthe particular ionized species. The positive voltage 213 may be appliedbetween the first electrode 205 and the second electrode 207. The secondelectrode 207 may be connected to a reference potential 215. As thesecond region 206 becomes depleted of charge carriers, the second region206 can acquire a net positive charge.

Prior to application of the positive voltage 213 and movement of theoxygen vacancies, the first region 204 (e.g., TiO_(2-X)) contains arelatively high concentration of oxygen vacancies (e.g., is in anoxidized state), with its stoichiometry being very close to TiO₂.Therefore, the TiO_(2-X) region 204 is a relatively good ionicconductor. The TiO₂ region 206 however, is depleted of oxygen vacancies.That is, the TiO₂ region 206 is essentially intrinsic, and there arevery few dopants in the lattice. As such, the transition between theTiO_(2-X) region 204 and the TiO₂ region 206 can act as a Schottkybarrier.

Application of the positive voltage 213 across the GCIB-treatedresistance device 202, with the polarity shown in FIG. 2A, causes theTiO_(2-X) region 204 to become an anode of an electrochemical cell. TheTiO_(2-X) region 204 is oxidized by the electrical energy input. Oxygenvacancies are driven out of the TiO_(2-X) region 204 and into the TiO₂region 206, as indicated by arrow 219, thereby reducing the TiO₂ region206.

Since only a relatively small number of oxygen vacancies are pushed outof the TiO_(2-X) region 204, the effect on the relatively goodelectrical conductivity of the TiO_(2-X) region 204 is fairly small.However, the electrical conductivity of the TiO₂ region 206 increasesdramatically, since it is changed from having a very low concentrationof oxygen vacancies in the molecular structure (e.g., essentially nooxygen vacancies), to a molecular structure (e.g., lattice) having someoxygen vacancies. In this manner, both the TiO_(2-X) region 204 and theTiO₂ region 206 are conducting, creating the low resistive state of theGCIB-treated resistance device 202.

The magnitude of the applied voltage used to change resistance states ofthe GCIB-treated resistance device may depend on the thicknesses andmaterials of the active region and/or particular portions thereof.According to one or more embodiments, a voltage magnitude in the rangeof 2.0 to 2.5 volts is used to effect resistance state changes.

FIG. 2B is a perspective view of a GCIB-treated resistance devicebeginning operated to a second resistance state in accordance with oneor more embodiments of the present disclosure. To obtain a highresistance state, positively charged donors, such as oxygen vacancies inTiO₂, are driven from the TiO₂ region 206 into the TiO_(2-X) region 204by application of a reverse-polarity voltage 217. When thereverse-polarity voltage 217 exceeds the threshold electric field forthe drift of the particular ionized species (e.g., a vacancy), TiO₂region 206 loses its net positive charge and again becomes chargeneutral.

The TiO_(2-X) region 204 still contains a relatively high concentrationof oxygen vacancies, and is therefore still a reasonably good conductor.Upon application of the reverse-polarity voltage 217, the TiO_(2-X)region 204 now becomes a cathode of the electrochemical cell. The TiO₂region 206 is oxidized towards intrinsic TiO₂, driving the oxygenvacancies out of TiO₂ region 206 and back into the TiO_(2-X) region 204(which is thereby reduced), as shown by arrow 223. The effect on theconductivity of the TiO_(2-X) region 204 is relatively small, but theconductivity of the TiO₂ region 206 decreases dramatically as oxygenvacancy concentration approaches zero.

If the higher resistance state of the GCIB-treated resistance device isdesignated as being a logical value “0,” and the lower resistance stateis designated as being a logical value “1,” then the GCIB-treatedresistance device can function as a non-volatile memory device since theresistance state persists after the voltage applied to change theresistance states is removed. Referring again to FIG. 1, to readinformation stored in the GCIB-treated resistance device can function asa non-volatile memory device, an interrogation voltage may be applied tothe GCIB-treated resistance device and the current flowing therethroughmay be measured. FIG. 3A is a diagram illustrating one example physicallayout of a GCIB-treated resistance device in accordance with one ormore embodiments of the present disclosure. According to the one or moreembodiments, a GCIB-treated resistance device can be fabricated to havean active region width (W) of approximately 2 nm, and an active regiondepth (including the TiO_(2-X) region 304 and the TiO₂ region 306) ofapproximately 3 nm. However, particular embodiments of the presentdisclosure are not limited to the afore-mentioned dimensions, and may befabricated with larger or smaller feature dimensions. The TiO₂ region306, adjacent the second electrode 307, can initially be nearlystoichiometric TiO₂ and thus initially be highly resistive. TheTiO_(2-X) region 204, adjacent the first electrode 305 can initially behighly oxygen deficient, and thus this region is initially highlyconductive.

FIG. 3B is an equivalent circuit diagram illustrating variableresistance of a GCIB-treated resistance device in accordance with one ormore embodiments of the present disclosure. The variable resistor 302shown in FIG. 3B corresponds to the physical layout of a GCIB-treatedresistance device shown in FIG. 3A. The total resistance R of theGCIB-treated resistance device is the sum of the resistance of theTiO_(2-X) region 304 (R1) plus the resistance of the TiO₂ region 306(R2). As described above, the change to the oxygen vacancy concentrationof the TiO_(2-X) region 304 due to application of voltage across theGCIB-treated resistance device is relatively small, thus the resistanceR1 remains relatively constant. The change to the oxygen vacancyconcentration of the TiO₂ region 306 due to application of voltageacross the GCIB-treated resistance device is relatively large, thus theresistance R2 changes dramatically (e.g., from effectivelynon-conducting to conducting). Thus R1 is represented in FIG. 3B as a(substantially) fixed resistor, and R2 is represented by a variableresistor 302.

FIGS. 4A-7E illustrate methods for forming a GCIB-treated resistancedevice according to one or more embodiments of the present disclosure.FIG. 4A is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device after deposition of a conformal oxidefilm in accordance with one or more embodiments of the presentdisclosure. According to one or more embodiments of the presentdisclosure, a substrate 442 is formed upon a bottom (e.g., second)electrode 440.

The substrate 442 may be formed of silicon (Si), silicon dioxide (SiO₂),silicon carbide (SiC), or other dielectric material. The bottomelectrode 440 may be formed of a metal or a metal oxide which forms aSchottky contact with an oxide. If the oxide film 446 is formed of ann-type oxide (e.g., TiO₂), then the bottom electrode 440 may be formedof a material having a relatively high work function, for example,platinum (Pt), ruthenium (Ru), iridium (Ir), iridium oxide (IrOx), amongothers. If the oxide film 446 is formed of a p-type oxide (e.g.,nitrogen oxide (NiO)), then the bottom electrode 440 may be formed of amaterial having a lower work function (e.g., titanium (Ti) or silver(Ag)), for example.

The substrate 442 may be formed to have a feature that will contain theactive region of the GCIB-treated resistance device (e.g., opening, via)therein that is oriented approximately perpendicular to the bottomelectrode 440 surface, or the feature may be formed in the substrateafter the substrate is deposited, such as by masking and etching. Thefeature may, or may not, be of a high aspect ratio. According to one ormore embodiments, the feature is fabricated to be 20 nm in depth, orless.

After formation of the feature, and according to one or moreembodiments, a conformal oxide film 446 is formed within and over thefeature and substrate 442. The oxide film may be formed of a materialhaving a lower electron conductivity, a higher oxygen conductivity, andat least two resistance states. The oxide film may be a metal oxidefilm, such as a thin film of TiOx, NiOx, CuOx, AlOx, and/or a nitridematerial (e.g., metal nitrides such as aluminum nitride). For example,the oxide film may be a transition metal oxide, such as nickel oxide(NiO), cesium dioxide (CsO₂), a vanadium oxide (VO₂ or V₂O₅), niobiumoxide (Nb₂O₅), a titanium oxide (TiO₂ or Ti₂O₃), tungsten oxide (WO₃),tantalum oxide (Ta₂O₅) or zirconium oxide (ZrO₂). However, embodimentsof the present disclosure are not limited by the aforementioned oxides,and the active region may be formed of oxides, nitride, and combinationsthereof, among others.

FIG. 4B is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device after removal of a portion of theconformal oxide film in accordance with one or more embodiments of thepresent disclosure. FIG. 4B illustrates removal of the portion of theconformal oxide film 446 outside the feature, such as bychemical-mechanical planarization/polishing (CMP) or other etch backprocess.

FIG. 4C is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device after gas cluster ion beam (GCIB)infusion of the oxide film in accordance with one or more embodiments ofthe present disclosure. Following removal of the portion of theconformal oxide film 446 outside the feature, the remaining oxide film446 (e.g., inside the feature in the substrate 442) is exposed to a GCIBto infuse a first (e.g., top) portion of the oxide film 446.

The fabrication methods of the present disclosure take advantage ofcertain characteristics of GCIB technology to create ionic gradients.GCIB treatment results in a uniform material with a known ionic (e.g.,oxygen) profile, to a well-controlled depth. The GCIB treatment may beoxidizing, or reducing, depending on the starting metal oxide film 446.According to embodiments of the present disclosure, GCIB treatment isused to change the surface material to amorphous as part of theinteraction during oxidation, deposition, or reduction. Thus, CCIBtreatment can result in creation of a grain insensitive region since theGCIB process is self-limiting yielding a precisely-defined region (e.g.,area within an active region).

Oxygen, nitrous, ammonia, hydrogen, deuterium, combinations thereof, orother appropriate gases can be used for the GCIB treatment. Combinationsof gases can be selected to provide desired tuning of atomic ratios andvacancy concentrations. According to one or more embodiments of thepresent disclosure, GCIB treatment is used to create gradients in themetal to oxygen ratio (e.g., Ti to oxide ratio, Ni to oxide ratio)within a scale of 1 to 50 nm. According to some embodiments, GCIBtreatment is used to create gradients in the metal to oxygen ratiowithin a range of from about 5 to about 20 nm. According to otherembodiments, GCIB treatment is used to create gradients in the metal tooxygen ratio within a range of from about 5 to about 15 nm.

Exposure to the GCIB results in the formation of a first region 448(e.g., first portion of the active region) and a second region 447(e.g., second portion of the active region), such as by causing oxygenvacancies to move from the first region 448 to the second region 447.For example, the homogeneous metal oxide film 446 can have an initialconcentration of oxygen vacancies. GCIB exposure can result in causing anumber of particular oxygen vacancies to move from their initiallocation in the molecular structure (e.g., lattice) of the metal oxidefilm 446, thereby forming two regions—a first region 448 having aresultant depleted oxygen vacancy concentration, and a second region 447having a resultant enhanced oxygen vacancy concentration.

The wafer surface remains at room temperature during GCIB treatment,therefore the methods of fabrication presently disclosed are not limitedto thermally stable pattern materials. For example, a photo resist couldbe used to maintain a self-aligned structure (e.g., application of amask for formation of the feature within dielectric 442 within which theactive region is formed, deposition of the oxide material only withinthe feature, GCIB treatment, followed by removal of the photo resistmaterial).

FIG. 4D is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device 402 after top electrode formation inaccordance with one or more embodiments of the present disclosure. Afterformation of the first region 448 and the second region 447, a top(e.g., first) electrode 450 can be formed adjacent the first region 448.The top electrode 450 may be formed of a metal or a metal oxide thatforms an ohmic contact with an oxide. For example, the top electrode 450may be formed of platinum (Pt), ruthenium (Ru), iridium (Ir), iridiumoxide (IrO_(X)), among others.

According to one or more embodiments of the present disclosure, materialused to form the active region of the GCIB-treated resistive device andthe dopant species are chosen such that the drift of the ions (orvacancies) within the portions of the active region is possible but doesnot easily occur to ensure that the GCIB-treated resistive device willremain in whatever resistive state it is programmed for a period of time(e.g., many years at room temperature after the operating electric fieldhas been removed). The metal oxide film can, for example, be formed by aphysical vapor deposition (PVD) process (e.g., sputtering), an atomicmaterial deposition (ALD) process, or a chemical vapor deposition (CVD)process, among other methods.

The ionic dopant species may be impurity atoms such as hydrogen or someother cation species, (e.g., alkali) or transition metals that act as anelectron donor for the first portion of the active region.Alternatively, the dopant species may be anion vacancies, which arecharged and therefore are also donors for a molecular lattice. It isalso possible to drive anionic species between regions of the activeregion, one region becoming electron acceptors (e.g., hole donors).

The active region material may be a metal oxide film (less than about 50nm thick), and is in many cases nanocrystalline, nanoporous oramorphous. The mobility of the dopant species in such nanostructuredmaterials is much higher than in a bulk crystalline material, sincediffusion can occur through grain boundaries, pores, or through localstructural imperfections in an amorphous material. Also, because of thestructure of the metal oxide film, the amount of time required to moveenough dopant species into or out of a particular region of the metaloxide film to substantially change its conductivity can be rapid. Forexample, the time t required for a diffusive process can vary as thesquare of the distance covered. Therefore, the time to diffuse adistance of one nanometer is about one-millionth of the time to diffuseone micrometer.

The material of the active region may be (after fabrication is complete)contacted on either side by metal electrodes or wires. The contact ofthe metal to the material of the active region can deplete thesemiconductor of free charge carriers, such that the material ofparticular portions of the active region can have a net charge thatdepends on the identity of the dopant species, which is positive in thecase of donors and negative in the case of acceptors. Themetal-semiconductor contact regions electrically resemble Schottkybarriers. The traditional description of a metal-semiconductor Schottkybarrier may be modified with respect to the GCIB-treated resistivedevice of the present disclosure by the fact that the materials arestructured at the nanometer scale, and so the structural and electricalproperties are not averaged over the large distances over which theoryof semiconductor-metal contacts have been developed.

Conduction of electrons through the active region material is viaquantum mechanical tunneling of the electrons. When a significant numberof dopant species have been infused into the semiconducting material ofone portion of the active region (e.g., by GCIB discussed below), thewidth and perhaps the height of the tunneling barrier are diminished bythe potential of the charged species. This results in an increase of theconductivity through the active region (lower resistive state).

As noted above, the material of the active region (e.g., metal oxide)has certain properties that are useful in the practice of the presentdisclosure. One of these properties of the material is that it is a weakionic conductor. The definition of a weak ionic conductor is based onthe application for which a GCIB-treated resistive device is designed.The mobility and the diffusion constant for a species in a lattice aredirectly proportional to one another, via the “Einstein relation.” Thus,if the mobility of ionized species in a lattice is very high, so is thediffusion constant. In general, it is desired for a switching device,such as a GCIB-treated resistance device, to stay in a particular state(e.g., resistive state) for an amount of time that may range from afraction of a second to years, depending on the application.

Accordingly, a particular GCIB-treated resistance device can befabricated to have an active region material with a diffusion constantlow enough to ensure the desired level of stability, and to avoidinadvertently switching the device from one resistive state to anothervia ionized species diffusion, rather than by intentionally setting thestate of the GCIB-treated resistance device with a applied voltagesignal. Therefore, a “weak ionic conductor” is one in which the ionmobility, and thus the diffusion constant, is small enough to ensure thestability of the various resistance states of the GCIB-treatedresistance device for as long as necessary under the conditions ofintended use. “Strong ionic conductors” would have large ionized speciesmobilities and thus would not be stable against diffusion.

FIG. 5A is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device after growth of a metal oxide film inaccordance with one or more embodiments of the present disclosure.According to one or more embodiments of the present disclosure, asubstrate 542 is formed upon a bottom (e.g., second) electrode 540. Thesubstrate 542 may be formed to have a feature (e.g., opening, via)therein approximately perpendicular to the bottom electrode surface, orthe feature may be formed in the substrate after the substrate isdeposited, such as by masking and etching. The feature may, or may not,be of a high aspect ratio. According to one or more embodiments, thefeature is fabricated to be about 20 nm in depth, or less.

After formation of the feature to contain the active region of theGCIB-treated resistance device, and according to one or moreembodiments, an oxide film 346, such as a metal oxide film (e.g., TiOx,NiOx, CuOx) is formed within the feature, such as by being grown withinthe feature from the bottom up.

FIG. 5B is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device after removal of a portion of the grownmetal oxide film in accordance with one or more embodiments of thepresent disclosure. FIG. 5B illustrates removal of the portion of thegrown metal oxide film 546 extending outside the feature, such as byCMP, or other etch back process, to arrive at an intermediate structuresimilar to that shown in FIG. 4B. The subsequent process, shown in FIGS.5C and 5D, can be as described with respect to FIGS. 4C and 4Drespectively.

FIG. 5C is a cross-sectional view illustrating a process for forming aresistance device after GCIB treatment of the metal oxide film inaccordance with one or more embodiments of the present disclosure. Afterremoval of the portion of the grown metal oxide film 546 extendingoutside the feature, the remaining metal oxide film 546 (e.g., insidethe feature in the substrate 542) is exposed to a GCIB to form a first(e.g., top) portion of the grown metal oxide film 546. The GCIBtreatment may be oxidizing, or reducing, depending on the starting metaloxide film 546. Oxygen, nitrous, ammonia, hydrogen, deuterium,combinations thereof, or other appropriate gases can be used for theGCIB treatment. Combinations of the gases can be selected to providedesired tuning of atomic ratios and vacancy concentrations. Exposure tothe GCIB results in the formation of a first region 548 and a secondregion 547, based on oxygen vacancy concentration, the first region 548having an oxygen vacancy concentration depleted by the GCIB treatment,and the second region 547 having an oxygen vacancy concentrationenhanced by the GCIB treatment.

FIG. 5D is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device 502 after top electrode definition inaccordance with one or more embodiments of the present disclosure. Afterformation of the first region 548 and the second region 547, a top(e.g., first) electrode 550 is formed adjacent the first region 548.

FIG. 6A is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device after partial growth of a metal oxidefilm in accordance with one or more embodiments of the presentdisclosure. FIG. 6A depicts a fabrication method similar to that shownin FIG. 5A, with the exception that the active region metal oxidematerial 646 is partially grown so that it does not extend outside thefeature formed in substrate 642. That is, the active region metal oxidematerial 646 can be fully contained within the feature formed insubstrate 642 and on electrode 640. As one having ordinary skill in theart will appreciate, partially growing the active region material withthe feature eliminates the necessity for a removal process step, such asthat illustrated between FIGS. 5A and 5B. Partial growth of the metaloxide material 646 within the feature of substrate 642 results in anactive region somewhat recessed in the feature.

FIG. 6B is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device after GCIB treatment of the partiallygrown metal oxide film in accordance with one or more embodiments of thepresent disclosure. Due to the anisotropic nature of the GCIB treatment,GCIB exposure can still be effective to form the initial active regionmaterial 646 to form a first region 649 and a second region 647, basedfor example on oxygen vacancy concentration, as has been previouslydescribed.

FIG. 6C is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device 602 after top electrode definition inaccordance with one or more embodiments of the present disclosure. Afterformation of the first region 649 and the second region 647, a top(e.g., first) electrode 650 is formed adjacent the first region 649 suchthat the electrode 650 extends into the feature as shown in FIG. 6C.

FIG. 7A is a cross-sectional view illustrating a process for forming aGCIB-treated resistance memory including a recessed active region afterpartial growth of a metal oxide film in accordance with one or moreembodiments of the present disclosure. GCIB is used to define one ormore atomically-tuned zones (e.g., active regions with particular oxygenvacancy characteristics) recessed within the feature used to contain aGCIB-treated resistive device (e.g., in the middle of a GCIB-treatedresistive device) by partially growing the active region material,treating by GCIB to form two regions therein, and then capping theactive region with a deposited metal oxide. FIG. 7A is similar to FIG.6A, showing partial growth of an active region metal oxide material 646such that it does not extend outside the feature formed in substrate742. That is, the active region metal oxide material 746 is fullycontained within the feature formed in substrate 742 on electrode 740.According to one or more embodiments, the active region metal oxidematerial 746 may be recessed more, or less, than that of active regionmetal oxide material 646 shown in FIG. 6A.

As previously discussed with respect to FIG. 6A, one having ordinaryskill in the art will appreciate that partially growing the activeregion metal oxide material 746 completely within the feature formed insubstrate 742 and on electrode 740 eliminates the necessity for aremoval process step, such as that illustrated between FIGS. 5A and 5B.Partial growth of the metal oxide material 746 within the feature ofsubstrate 642 results in an active region recessed in the feature.

FIG. 7B is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device after GCIB treatment of the partiallygrown metal oxide film in accordance with one or more embodiments of thepresent disclosure. As discussed with respect to FIG. 6B, due to theanisotropic nature of the GCIB treatment, GCIB exposure is effective toform the initial active region material 746 so as to form a first region752 and a second region 747, the regions being defined, for example,based on oxygen vacancy concentration.

FIG. 7C is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device after deposition of a conformal metaloxide film in accordance with one or more embodiments of the presentdisclosure. After partial growth of the active region metal oxidematerial 746, and GCIB exposure thereof to form a first 752 and second747 region within the recessed active region, according to one or moreembodiments, a conformal oxide film 754 is formed over the recessedactive region atop the first region 752 and substrate 742. The conformaloxide film 754 may be formed of the same, or different, material as theactive region material 746 initially grown within the feature. Forexample, the conformal oxide film may be a metal oxide film, such asTiOx, NiOx, CuOx. The thin metal oxide film can, for example, be formedby atomic material deposition (ALD), or chemical vapor deposition (CVD),among other methods.

FIG. 7D is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device after removal of a portion of theconformal metal oxide film 754 in accordance with one or moreembodiments of the present disclosure. FIG. 7D illustrates removal ofthe portion of the conformal metal oxide film 754 outside the feature,such as by chemical-mechanical planarization/polishing (CMP) or otheretch back process. According to one or more embodiments, and unlike thatwhich was shown and described with respect to FIG. 2C, the conformalmetal oxide film 754 is not further subjected to a GCIB treatment. Onehaving ordinary skill in the art will appreciate that formation of theconformal metal oxide film 754 over the first region 752 results in aGCIB-treated resistive device having a fully recessed active regionwithin the feature. The feature is nominally configured to have adimension perpendicular to the region boundaries in the range of 20 nmor less.

FIG. 7E is a cross-sectional view illustrating a process for forming aGCIB-treated resistance device 702 after top electrode definition inaccordance with one or more embodiments of the present disclosure. Afterremoval of a portion of the conformal metal oxide film 754, a top (e.g.,first) electrode 750 is formed adjacent the conformal metal oxide film754.

Burying the active region (e.g., first 747 and second 752 regions) ofthe GCIB-treated resistive device can have the advantages of maintainingstable oxygen concentrations in the active region. In addition, certainthermal considerations associated with joule heating of the activeregion benefit from burying the active region, since such aconfiguration isolates the active region within the dielectric substrate742.

Because of the cyclic capability of GCIB, the disclosed fabricationmethods may also be used to make laminates of differential atomic ratiosto optimize certain characteristics of the GCIB-treated resistivedevice. For example, a laminate may be formed by repetitivedeposition/growth of an oxide material followed by GCIB.

The disclosed methods can also be applied to electrolyte cell memoryfabrication, where a thin oxide is used to act as a regulating barrierfor ion and electron movement during cycling of the memory cell. Forexample, a Ge-based electrolyte memory cell may be fabricated byformation of a GeOx material (treated by GCIB) with a Ge material formedatop the GeOx material.

FIG. 8 illustrates an example stacked RRAM structure having multipleresistance states that can be implemented as the GCIB-treated resistancedevice in FIG. 1 according to one or more embodiments of the presentdisclosure. One having ordinary skill in the art will appreciate thatmultiple resistance states may be achieved by coupling multipleGCIB-treated resistance devices in series and/or parallel combinations.FIG. 8 shows a first GCIB-treated resistance device 802-1 and a secondGCIB-treated resistance device 802-2 connected in series. FirstGCIB-treated resistance device 802-1 may be fabricated to have firstresistance characteristics, and second GCIB-treated resistance device802-2 may be fabricated to have second resistance characteristics.

The overall resistance, R, of the series connected GCIB-treatedresistance device is the sum of the resistances of the GCIB-treatedresistance device. One skilled in the art can extrapolate the method bywhich an active region may be buried within a feature as shown withrespect to FIGS. 7A-E, to form GCIB-treated resistive devices havingmultiple active regions. With appropriate control logic, it is thereforepossible to achieve four discrete values of total resistance, R, therebyenabling fabrication of a multilevel memory cell. Embodiments of thepresent disclosure are not limited to four resistance states as shownand described with respect to FIG. 8. One having ordinary skill in theart will appreciate that more or fewer states may be obtained by variouscombinations of GCIB-treated resistance device, having various low andhigh resistance states.

Although specific embodiments have been illustrated and describedherein, those of ordinary skill in the art will appreciate that anarrangement calculated to achieve the same results can be substitutedfor the specific embodiments shown. This disclosure is intended to coveradaptations or variations of one or more embodiments of the presentdisclosure. It is to be understood that the above description has beenmade in an illustrative fashion, and not a restrictive one. Combinationof the above embodiments, and other embodiments not specificallydescribed herein will be apparent to those of skill in the art uponreviewing the above description. The scope of the one or moreembodiments of the present disclosure includes other applications inwhich the above structures and methods are used. Therefore, the scope ofone or more embodiments of the present disclosure should be determinedwith reference to the appended claims, along with the full range ofequivalents to which such claims are entitled.

In the foregoing Detailed Description, some features are groupedtogether in a single embodiment for the purpose of streamlining thedisclosure. This method of disclosure is not to be interpreted asreflecting an intention that the disclosed embodiments of the presentdisclosure have to use more features than are expressly recited in eachclaim. Rather, as the following claims reflect, inventive subject matterlies in less than all features of a single disclosed embodiment. Thus,the following claims are hereby incorporated into the DetailedDescription, with each claim standing on its own as a separateembodiment.

What is claimed is:
 1. A method for forming a resistive device,comprising: forming an active region material on a first electrode;causing a greater concentration of an ionic dopant species in a firstregion of the active region material relative to a second region of theactive region material by exposure of the active region material to agas cluster ion beam (GCIB); and forming a second electrode over theGCIB-exposed first region of the active region material.
 2. The methodof claim 1, wherein the first region is located nearer the secondelectrode than is the second region.
 3. The method of claim 1, whereincausing the greater concentration of the ionic dopant species in thefirst region includes forming the active region material to have agreater number of ionic vacancies in the first region than in the secondregion of the active region material.
 4. The method of claim 3, whereinthe ionic vacancies are oxygen vacancies.
 5. The method of claim 3,wherein the ionic vacancies are nitride vacancies.
 6. The method ofclaim 3, the method further comprising inputting electrical energy tothe active region material sufficient to move at least a portion of theionic dopant species from the first region so as to cause an increase ina number of ionic vacancies in the second region of the active regionmaterial.
 7. The method of claim 1, the method further comprisinginputting electrical energy to the active region material sufficient tomove at least a portion of the ionic dopant species with respect to thefirst region so as to cause a change in conductivity of the activeregion material.
 8. The method of claim 7, wherein inputting electricalenergy to the active region material includes inputting electricalenergy to the active region material sufficient to cause a greaterchange in conductivity of the second region of the active regionmaterial relative to a change in conductivity of the first region of theactive region material.
 9. The method of claim 1, wherein causing thegreater concentration of the ionic dopant species in the first regionincludes forming a gradient of the ionic dopant species in the firstregion of the active region material.
 10. The method of claim 1, whereinthe active region material is an oxide material.
 11. The method of claim1, wherein the material is a metal nitride material.
 12. A resistivedevice produced according to the process of claim
 1. 13. A method forforming a resistive device, comprising: forming a material over a firstelectrode, the material having an ionic dopant species; causing a numberof the ionic dopant species to move within the material by exposure ofthe material to a gas cluster ion beam (GCIB) to an extent that changesresistivity within the portions of the material relative to one another;and forming a second electrode over the material.
 14. The method ofclaim 13, wherein causing the number of ionic dopant species to moveincludes causing at least one of hydrogen, an alkali cation, and atransition metal that acts as an electron donor to move.
 15. The methodof claim 13, wherein the material is a nitride material.
 16. A methodfor foaming a resistive device, comprising: forming, over a firstelectrode, a material having an ionic dopant species at initiallocations in a molecular structure of the material; causing, in a firstregion of the material, a number of the ionic dopant species to movefrom the initial locations to a second region of the material byexposure to a gas cluster ion beam (GCIB); and forming a secondelectrode over the first region of the material.
 17. The method of claim16, wherein the material is an oxide material.
 18. The method of claim17, wherein the oxide material includes a material of the groupcomprising: titanium oxide, nickel oxide, copper oxide, and aluminumoxide.
 19. The method of claim 17, wherein the oxide material is ann-type oxide material, and wherein the lower electrode includes at leastone of platinum (Pt), ruthenium (Ru), iridium (Ir), and iridium oxide(IrO_(x)).
 20. The method of claim 17, wherein the oxide material is ap-type oxide material, and wherein the lower electrode includes at leastone of titanium (Ti), silver (Ag), nickel oxide (NiO), cesium dioxide(CsO₂), vanadium oxide (VO₂), vanadium oxide (V₂O₅), niobium oxide(Nb₂O₅), titanium oxide (TiO₂), titanium oxide (Ti₂O₃), tungsten oxide(WO₃), tantalum oxide (Ta₂O₅) and zirconium oxide (ZrO₂).